Expand C API coverage: reset/backup/blob I/O/error reporting

[colemancda]

Summary

• Add sqlite3_reset, sqlite3_clear_bindings, and sqlite3_expanded_sql to Statement, allowing prepared statements to be reused instead of only finalized.
• Let custom scalar and aggregate functions report failures back to SQLite via sqlite3_result_error (block/final closures can now throw).
• Switch Connection's close path to sqlite3_close_v2 and add sqlite3_get_autocommit, sqlite3_busy_timeout, and sqlite3_interrupt wrappers.
• Add an IncrementalBlob wrapper around the incremental BLOB I/O API (sqlite3_blob_open/read/write/bytes/close/reopen).
• Add a Backup wrapper around the online backup API (sqlite3_backup_init/step/finish/remaining/pagecount).
• Add WAL support: Connection.journalMode / setJournalMode(_:) (via PRAGMA journal_mode, since there's no dedicated C API for it) and walCheckpoint(mode:database:) / setWALAutoCheckpoint(pageCount:) wrapping sqlite3_wal_checkpoint_v2 / sqlite3_wal_autocheckpoint.

Out of scope for this PR: sqlite3_exec (redundant with the existing Connection.run convenience), and update/commit/rollback hooks (sqlite3_update_hook, etc.) — those need a retained callback box tied to Connection's lifetime, which the current struct doesn't have room for without a larger design change. sqlite3_wal_hook falls in the same bucket and is likewise deferred.

Test plan

• swift test — 124 tests pass, including new suites IncrementalBlobTests, BackupTests, and WALTests, and new cases in ConnectionStatementTests / FunctionEdgeCaseTests.

[github-code-quality]

Code Coverage Overview

Languages: Swift

Swift / code-coverage/llvm-cov

The overall coverage in the feature/expand-c-api... branch is 96%. The coverage in the master branch is 97%.

Show a code coverage summary of the most impacted files.

File	master 248ab01	feature/expand-c-api... 7b2b849	+/-
Sources/SQLite/Statement.swift	97%	94%	-3%
Sources/SQLite/...onnection.swift	96%	96%	0%
Sources/SQLite/Function.swift	98%	98%	0%
Sources/SQLite/...eFunction.swift	87%	88%	+1%
Sources/SQLite/Backup.swift	0%	85%	+85%
Sources/SQLite/WAL.swift	0%	88%	+88%
Sources/SQLite/...entalBlob.swift	0%	96%	+96%

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